Topband: Measuring Common Mode Chokes
jim.thom at telus.net
Fri Dec 20 01:19:40 EST 2019
Date: Wed, 18 Dec 2019 23:46:56 +0000
From: Chuck Hutton <charlesh3 at msn.com>
To: "topband at contesting.com" <topband at contesting.com>
Subject: Topband: Measuring Common Mode Chokes
<In the past, I have simply used my N2PK VNA to measure impedance of the choke by connecting the shield of the coax to the VNA ports.
<Recently I've been discussing common mode chokes with others who have a different methodology.
<They prefer to do a transmission test through the choke and report the "common mode rejection".
<This is done by placing a crossover cable between the VNA output and the choke. The choke output is connected in a normal fashion (center to center, shield to shield) to the VNA input.
<This does not seem ideal to me.
<First, the choke is being driven in differential mode rather than common mode.
<Second, the measurement depends on (varying) isolation between the coax center and shield. So it's not truly common mode rejection.
<Am I on thr right track?
<A handful of Googles has not netted me any clear summary of test methodology for reporting CMRR. I find a small number of tests reporting impedance.
## I measure the actual common mode current with a MFJ-854 clamp on RF ammeter. Good from less than 1ma to 3A. Just clamps around the 213-U coax.
Then when swapping CM chokes, any changes or improvements, or worsening is readily apparent at a glance. I measure In several places on the coax run, and all are well marked.
https://www.mfjenterprises.com/Product.php?productid=MFJ-854 Switchable In 5 ranges.
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